Make the draw a commit-reveal at a fixed, confirmed, quorum-agreed height with a public proof (B-73, RND-14 … RND-20, RND-33 … RND-37, DAT-61 … DAT-66, API-25, API-45, AUD-24 … AUD-27, AUD-37, ELC-15, SEC-56, ADR-24, ADR-25)

The seed was the hash of the first block seen after the close, so a miner holding tickets
could withhold blocks until one made them win, the active Electrum server could pick which
real block was used by not announcing another (a failed corroboration even moved the draw to
the next block), and the participant order was the server-only `broadcast_at` with no
tie-break, so nobody could recompute the result.

- Open: every round gets a 32-byte `server_seed`, stored only Fernet-encrypted with the
  master-xprv key (`server_seed_enc`), with its published `server_seed_hash`;
  `draw_confirmations` is copied from round_config. The plaintext column is written only by
  the reveal.
- Entering `drawing` (one transaction, `draw_committed` audit row): participants ordered by
  bet txid, each `draw_position`, `participants_hash` (sha256 of `txid:vout:address\n`
  lines; new `bet_vout`), `draw_height = tip_at_close + 1`. A tip of 0 keeps the round
  `closing` (`draw_baseline_tip_unknown`, once per round). All persisted, so a resumed draw
  reads rather than recomputes (the `_tick` branch itself is B-81).
- `_complete_draw` / `_wait_for_draw_block` replace `_wait_for_next_block`: wait for
  `draw_confirmations` blocks from `draw_height`, then `ElectrumListener.agreed_block_hash`
  asks every server, the active one included, for the header at that exact height (valid
  work, >= 2 other answers, strict majority of all); no agreement means the same height is
  asked again and `draw_header_corroboration_failed` is written once per distinct reported
  hash. `draw_stalled` now counts from the last tip advance, or from when the quorum started
  to fail. Seed = sha256(server_seed || block_hash || participants_hash), index = seed mod n.
- The winner, the seed and the reveal (`server_seed`, `winner_drawn` payload) are written in
  one transaction; a round closing with no participants reveals too. A round opened before
  this change (no commitment) is given a secret as it enters `drawing`.
- `GET /rounds/{id}/proof` (public, 404 `round_not_found`, translated in all 7 languages) and
  the new fields on `GET /rounds/current`; `draw_confirmations` in `/admin/config` and the
  /admin form; `server_seed_hash`/`draw_height` in `/admin/rounds`, never the secret.
- Alembic d3e8b15a7c42 (plain add_column / create_index, expression indexes untouched).
- Docs: the B-73 `*Stato:*`/`*Oggi:*` lines are gone, guides and CLAUDE.md describe the new
  draw. Ambiguities resolved in api.md: the proof's participants also carry `amount_sats`
  (RND-37 lists it); `seed` is decimal in the proof and hex in the audit row, as specified.

Tests changed because the spec changed: ElectrumListener.corroborate_header (asked only the
other servers about the block the active one announced) is replaced by agreed_block_hash and
its quorum tests ported; draw_winner takes the secret and the list fingerprint; the four
_wait_for_next_block scheduler tests are replaced by _wait_for_draw_block ones (fixed height,
confirmations, same-height retry, stall clock). New: test_draw_proof.py recomputes the winner
independently from the public proof, checks the plaintext secret is nowhere in the database,
the API or the log before the reveal, and that a second pass over a `drawing` round yields
the same winner. Mutation-checked: ordering by broadcast_at, an early reveal and asking the
tip instead of the draw height each fail tests.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/unit/test_rounds_service.py

@@ -288,3 +288,64 @@ async def test_cooldown_comes_from_the_round_that_closed(session_factory):
 
     async with session_factory() as session:
         assert await open_new_round_if_needed(session) is None  # still cooling down
+
+
+# --- B-73 (RND-34, DAT-61, DAT-64, SEC-56): the commitment made when a round opens --------
+
+
+async def test_a_new_round_commits_to_an_encrypted_secret_and_copies_draw_confirmations(session_factory):
+    """The hash is published, the secret is stored only encrypted (and decrypts to the
+    preimage of the hash), the plaintext column stays empty, and the number of confirmations
+    is copied from the config at open — an edit applies from the next round."""
+    import hashlib
+
+    from app.wallet.keystore import decrypt_round_seed
+
+    async with session_factory() as session:
+        (await session.scalars(select(RoundConfig))).one().draw_confirmations = 5
+        await session.commit()
+    async with session_factory() as session:
+        round_ = await open_new_round_if_needed(session)
+        await session.commit()
+        round_id = round_.id
+
+    async with session_factory() as session:
+        round_ = await session.get(Round, round_id)
+        assert round_.draw_confirmations == 5
+        assert round_.server_seed is None  # DAT-64: no plaintext before the reveal
+        assert round_.server_seed_enc and round_.server_seed_hash
+        seed = decrypt_round_seed(round_.server_seed_enc)
+        assert len(bytes.fromhex(seed)) == 32
+        assert hashlib.sha256(bytes.fromhex(seed)).hexdigest() == round_.server_seed_hash
+        assert seed not in round_.server_seed_enc  # an encrypted token, not the secret in disguise
+        (await session.scalars(select(RoundConfig))).one().draw_confirmations = 9  # edited mid-round
+        await session.commit()
+        assert (await session.get(Round, round_id)).draw_confirmations == 5
+
+
+async def test_every_round_gets_its_own_secret(session_factory):
+    seeds = set()
+    for _ in range(3):
+        async with session_factory() as session:
+            round_ = await open_new_round_if_needed(session)
+            round_.status = "closed"
+            round_.closed_at = datetime.now(timezone.utc) - timedelta(days=1)
+            seeds.add(round_.server_seed_hash)
+            await session.commit()
+    assert len(seeds) == 3
+
+
+async def test_the_secret_cannot_be_decrypted_with_another_key(session_factory, monkeypatch):
+    """SEC-56: a copy of the database without XPRV_ENCRYPTION_KEY does not reveal it."""
+    from cryptography.fernet import Fernet, InvalidToken
+
+    from app.config import settings
+    from app.wallet.keystore import decrypt_round_seed
+
+    async with session_factory() as session:
+        round_ = await open_new_round_if_needed(session)
+        await session.commit()
+        token = round_.server_seed_enc
+    monkeypatch.setattr(settings, "xprv_encryption_key", Fernet.generate_key().decode())
+    with pytest.raises(InvalidToken):
+        decrypt_round_seed(token)